Fluid-fluid explosive self-mixing, sometimes referred to as "steam" explosions in the particular circumstances where water is involved, is a common and well known hazard and phenomenon in industry, particularly the foundry industry. Such an explosion can occur, for example, when a hot molten metal falls into a bath of water or on damp earth. The violence of these explosions can be major. In the aluminum industry there have been accidents where more than 100 workmen have been killed and a whole foundry destroyed.
Such explosions are caused primarily by bringing a hot fluid -- e.g., hot molten metal, salt, or glass -- into sudden and close contact with a cold vaporizable fluid -- e.g., water, industrial solvents, or heat transfer fluids -- that have a high vapor pressure, say on the order of hundreds of atmospheres, when they are at the temperature of the hot fluid. Under these circumstances, an explosion frequently occurs if some kind of trigger pressure pluse forces the fluids into contact with one another. However, the explosion may not need to be triggered in all cases. In other instances, minor triggers -- e.g., delayed supercritical boiling, mechanical motion, and even bubbles of one fluid trapped by the other in the bottom of a container -- may cause the explosion. At any rate, once a rapid mixing begins, it is likely to continue until a fair fraction of the two fluids have exchanged almost all their heat and energy. Apparently, the mixing is self-driven, and fluid instabilities allow one fluid to mix into the other in extremely small particles, as small as a micron in size, so that the heat exchange occurs in milliseconds or less time. The pressure of the explosion is limited by the vapor or "steam" pressure at the temperature of the hot fluid. This may be 5,000 to 10,000 psi for molten metals and water.
A similar fluid-fluid self-mixing explosion can occur between a cold fluid like cryogenic gases and a hot fluid like water or any other room temperature fluid -- e.g., oil, gasoline, or alcohol. The driving pressure is the gas pressure of the cryogenic gas at liquid density and room temperature. When the temperature difference between the cryogenic gas and the room temperature fluid is not as large as, say, molten steel and water, the explosions will not be so violent and may require a larger trigger in order to be initiated.
A cryogenic fluid-fluid self-mixing explosion is greatly feared in the situation where a tanker ship carrying liquid natural gas -- i.e., liquid methane -- is involved in a collision or other accident. The water is the hot fluid, and liquid natural gas is the cryogenic cold fluid. In this situation even a very modest explosion could rip the ship apart, releasing the entire cargo of gas, which could then deflagrate with far worse consequences than the original fluid-fluid self-mixing explosion itself. For example, if a 100,000 ton tanker carrying liquid methane were to collide with another ship in circumstances causing a fluid-fluid self-mixing explosion, the explosion could conceivably be as large as the available energy difference between the liquid methane and the water -- i.e., equivalent roughly to 10,000 tons of a normal high-explosive. This, or a smaller explosion, could disperse the methane into the atmosphere. If the methane were then ignited when it reached the correct stoichiometric mixture with the oxygen in the atmosphere, the deflagration or detonation could be equivalent roughly to 1,000,000 tons of a normal high-explosive. There is obviously, therefore, considerable motivation to reduce the probability of such an event occurring within the harbor of a large city.
Many individuals have studied the possible explosive self-mixing of liquid natural gas and water and come to varying conclusions concerning its safety. These revolve around questions of supercritical boiling, the admixture of ethane, etc., but the fact remains that some fluid-fluid self-mixing explosions involving liquid natural gas have been created in the laboratory and in the field; the U.S. Coast Guard, for example, has conducted such experiments.
The one requirement for a fluid-fluid self-mixing explosion is that the fluids must come into intimate contact. If a gas film or gas barrier is interposed, the fluids will not explosively self-mix because the gas film pressure is too low. If a gas film is formed by the two fluids themselves -- e.g., if they are poured relatively slowly into each other -- it is conceivable that they will only boil and not explode. In this situation, the gas film must be formed at a rate greater than the rate at which the two fluids come into contact. Therefore, if something inhibits the formation of such a gas film -- like the property of the cold fluid to transiently come to the temperature of the hot fluid without boiling, the property called "superheat" -- the two fluids could come into contact without a gas film so that they might explosively self-mix. This is the property that has been proposed by Fauske to explain fluid-fluid explosive self-mixing.
On the other hand if regardless of the superheat criteria a pressure pluse strong enough to overcome the gas film is applied, the fluids will explosively self-mix. This has been shown for very hot molten metals and water. Thus, in the situation where a tanker ship is carrying liquid natural gas, there is a real danger of a fluid-fluid self-mixing explosion between the gas and the water because of the large amount of gas and the possible triggering effect of high pressures created during a collision.